You need to know all math up through Algebra II like the back of your hand. This includes basic geometry. You also need to have excellent reading / writing skills, since a short essay section has been added to the SAT, and there are of course writing mechanics and reading comprehension to worry about.

I suggest buying or borrowing an SAT test prep book. Do a practice test, then study hard in the areas you're weakest in. Saxon is good for math up through Alg. II, reading comprehension will improve as you read more books, and your writing just needs a massive amount of practice. If possible, write at least a couple short essays each week on topics you know a lot about, or feel strongly about. Have all your mistakes pointed out, then rewrite to fix the mistakes.

There are free programs on the internet and websites that offer free help too. I can only think of one but it is http://www.studyguidezone.com. Don't know how much help it will be but you could do your own search too.

We went to a big bookstore and flipped through all the ACT prep books they sold till we found the one that looked like it would give us the most value for the money. In the front there was a recommended schedule for studying up, and we followed it almost exactly, adapting slightly for our own situation.